Wowio has some neat stuff for kids and a pretty big selection. They're not free, but you can preview them online and buy them as PDFs.

Manybooks has free, public domain (i.e., old) books, also available in PDF. They don't have much stuff post-1920 because of copyright law, so that might not be too helpful when it comes to nonfiction, but they have lots of classics. I read a -lot- of books from there, so if you'd like some recommendations, I might be able to pull out a few.